fMapper - A FunctionMapper to resolve functions found in the expression.
It can be null, in which case no functions are supported for
this invocation. The ExpressionEvaluator must not hold on to
the FunctionMapper reference after returning from
parseExpression(). The Expression
object returned must invoke the same functions regardless of
whether the mappings in the provided
FunctionMapper instance change between calling
ExpressionEvaluator.parseExpression() and
Expression.evaluate().

Evaluates an expression. This method may perform some syntactic
validation and, if so, it should raise an ELParseException error if it
encounters syntactic errors. EL evaluation errors should cause an
ELException to be raised.